This invention relates to optical modulators and, more particularly, optical modulators using minimum-shift keying (MSK).
Minimum-shift keying (MSK) is a well-known modulation format in radio and microwave communications. See M. L. Doelz and E. T. Heald, Minimum-shift data communication system, U.S. Pat. No. 2,977,417, Mar. 28, 1961. It is also called fast frequency shift keying (Fast FSK). See R. deBuda, IEEE Trans. Commun. COM-20, p. 429 (1972). Here by MSK we refer to all continuous phase frequency shift keying (CPFSK) modulation schemes with a modulation index h=0.5. See J. B. Anderson, T. Aulin, and C.-E. Sundberg, Digital Phase Modulation (Plenum Press, New York,1986); C.-E. Sundberg, IEEE Communications Magazine, Vol. 24, p.25 (1986).
Compared with amplitude modulation, MSK in fiber optic based communications systems has a narrower power spectrum and suffers less nonlinear degradation by keeping the amplitude envelope constant. In spite of its many advantages, MSK has never been used in any commercial optical fiber communication systems mainly due to the difficulties in manufacturing reliable optical MSK transmitters and receivers. Today in optical fiber communications, the transmitted data is encoded in the intensity of the lightwave (known as on-off keying or OOK) instead of the phase.
We have implemented external electro-optical modulation in an MSK format for the first time. The MSK modulator of the invention employs a CLOCK section and a DATA section. In the CLOCK section, the input cw light source is carved into two clock pulse streams. One way to achieve this is to use a pair of phase modulators and a coupler to carve the input laser source into two clock pulse streams. The DATA section then uses another pair of phase modulators to appropriately modulate the phase of each clock pulse. Finally, a second coupler is used to add up the two streams in a desired manner, thus implementing the MSK transmission mode.